1. Field of the Invention
The subject matter of this invention relates generally to voltage regulators for three phase voltage transmission, distribution or utilization lines. The subject matter of this invention relates more particularly to the utilization of controlled static VAR generators for providing voltage regulation.
2. Description of the Prior Art
It is well known to utilize voltage regulators to stabilize the voltage at a particular terminal location on multi-phase electrical transmission, distribution or utility lines. It has been found that a transmission line may be represented by an apparent impedance which may be inductive. A varying load connected to the terminals to be regulated may cause the line current to vary. The varying line current produces a voltage drop across the impedance of the line which vectorially adds to the source voltage thus producing a terminal voltage which may be less than, equal to or greater than the source voltage, depending upon the many things including the effects of other components in the system. This observed phenomena becomes significant as the line current varies because the varying line current changes the voltage drop across the impedance of the line, thus changing the voltage at the terminals which supply power to the load. This characteristic is undesirable in a circuit which is to be regulated. A regulating apparatus may be provided at the terminals to compensate for the changes in the vectorially added source and inductive reactance voltage. It is generally well known that as load impedance decreases line current may become significantly larger, thus introducing a voltage drop in the supply line which subtracts from the terminal voltage of the load. It is undesirable to have load voltage vary in many instances. Consequently, a voltage regulator may be provided proximate to the terminals across which voltage is to be relatively constant. Typical AC regulators are described in the Control Engineers' Handbook edited by J. G. Truxal, 1958, Sections 11.18-11.21. Most popular schemes for voltage regulation utilize rotating synchronous condensers, tap-changing transformers, chokes or autotransformers in conjunction with voltage sensing device. These regulating devices have a number of disadvantages which include lossiness and slow response time. It has recently been found that reactive current can be supplied to or withdrawn from electrical conductors by utilizing a fixed capacitor and thyristor controlled inductor connected in parallel across the lines. The fixed capacitor provides current which leads the voltage across the line by 90.degree. and the thyristor controlled inductor provides current which lags the voltage across the line by 90.degree.. The inductive current is therefore 180.degree. out of phase with the capacitive current. By controlling the firing angle of the thyristor switch which in turn controls the inductor, the amount of reactive current and its phase can be shifted, depending upon the parameters of inductance and capacitance, for example. Such being the case, leading or lagging reactive current may be supplied to the line. This idea has been utilized extensively in the past in the form of VAR (volt amps reactive) generator circuits. These circuits have been utilized for power factor correction and flicker control. Examples of such circuits may be found in U.S. Pat. Nos. 3,987,360, issued Oct. 19, 1976 to M. B. Brennen et al; 3,999,117, issued Dec. 21, 1976 to Gyugyi et al; and 4,000,455, issued Dec. 28, 1976 to Gyugyi et al. These patents teach VAR generation. The conduction angle of the thyristor switch which controls the current flowing through the inductor of the VAR generator is determined by a static control system which is interconnected with the lines to be controlled. It would be advantageous if a VAR generator type circuit with associated control could be utilized in conjunction with polyphase electrical transmission systems to provide regulating voltage. The subject matter of the present invention was at least partially disclosed in a report filed July 2, 1976 for the Electric Power Research Institute, Inc. by L. Gyugyi and W. J. Lordeon, entitled Quarterly Report No. 2, RP 750-1 "Transmission System Static VAR Control."